A sonic/ultrasonic-assisted method for the compaction and injection of granular slurries and pastes in the subsurface

ABSTRACT

A compaction and/or hydro fracture generation device ( 10 ) comprises a compression device ( 11 ), a vibration generator device ( 12 ) arranged to generate vibrations in the ultrasonic frequency range, and a power source connected to the vibration generator.

This application regards a compaction device for efficient compaction and/or hydro fracture generation.

During the operation of the Badger Explorer, a drill tool of the type described in NO312110, a hole is drilled into a subsurface formation and the waste rock from the drilling process is deposited as a plug behind the advancing tool. Due to dilatancy that occurs when the formation is cut, the volume of the material that must be deposited behind the tool is greater than that which was cut. The waste material must be treated in such a manner that it recovers its original density and/or accommodation space for the expanded volume must be made within the formation.

A similar problem is faced by the mining industry in where mineral ores are removed from the subsurface, processed, and the waste rock is then used to backfill the subsurface tunnels and caverns from which it was excavated.

A similar problem is also encountered in the field of thick slurry injection in which produced solid wastes from hydrocarbon wells are re-injected into subsurface formations.

Simple compression of the waste will expel some of the fluids (liquids and gases) that are present in the pore space, but this will in many cases not be sufficient.

Compaction of mine back-fill using lower frequency vibrating compactors has been performed at surface for the mining industry, and has likely been used subsurface. Densification of surface deposits using an experimental low-power ultrasonic compactor for geotechnical ground improvement has also been reported (Towhata, I., Geotechnical Earthquake Engineering, Springer-Verlag 2008, pp 608).

The object of the invention is to provide a device and method for efficient compaction and/or hydro fracture generation which alleviates at least some of the above mentioned problems and challenges.

The object of the invention is achieved by means of the features of the patent claims.

The material which is to be compacted, for example for the applications described above, will in this description be referred to as “paste”. This term is meant to describe any material which is to be compacted or injected, for example in the underground.

A potential application of the described technology that does not rely on the increased ability to compact, but rather on improved injection performance, is in the field of hydro fracture and hydraulic stimulation, such as in wellbores to create small fractures (typically less than 1 mm), along which fluids such as gas, petroleum, uranium-bearing solution, and brine water may migrate to the well. By applying vibrations at the injection point during a stimulation, the skin friction of the well will be reduced and vibration during the emplacement of proppant will allow wider aperture and hence more productive stimulation.

The object of the invention is achieved by a combination of two mechanisms: compression and high frequency vibration.

It is well established that vibrating a dry or saturated granular mixture will cause a drop in the viscosity of the mixture. The decrease in the resistance to flow is related to the frequency of the vibrations, the size of the particles in the mixture, and the amplitude of the vibrations (Melosh, Journal of Geophysical Research, 1979).

High power (>10 W/cm2)-high frequency (>8 kHz) ultrasonic vibrations applied to a surface which is used for compacting, will thus fluidize the paste and allow pore fluids to be expelled until the paste has been compressed to its jamming point. Jamming is the physical process by which some materials, such as granular materials, glasses, foams, and other complex fluids, become rigid with increasing density. The density at which systems jam is determined by many factors, including the shape of their components, the deformability of the particles, frictional interparticle forces, and the degree of dispersity of the system. The overall shape of the jamming manifold may depend on the particular system.

The jamming point for the ultrasonically vibrated paste will occur at a higher density than the non-vibrated paste, leading to a higher final density of the material, for example in a compacted plug.

Fluidization and energizing of the paste will allow it to enter and widen small apertures and hydrofractures that would otherwise be too thin to accommodate the material.

In one embodiment a compaction device according to the invention comprises a compression device, a vibration generator device arranged to generate vibrations in the ultrasonic frequency range, and a power source connected to the vibration generator.

The compression device is a device that compresses the material, ie. the paste, to as compact condition as possible. The compression device can be any suitable device which is capable to compress the material in question, and is in one embodiment a moving piston plate. The piston can be a pneumatic or hydraulic piston with a plate mounted at its further end perpendicular to the movement direction of the piston. The extension of the piston towards the paste causes the compression of the paste.

The vibrator generator device is a device adapted to generate vibrations of a desired frequency in the paste in order to achieve the fluidization effect described above. In one embodiment the vibration generator device is a sonic or ultrasonically vibrating surface that is affixed to the moving piston-plate. The vibrations will then be transferred via the piston plate and then relayed into the paste, causing the desired vibrations in the paste. The vibration generator device may also be connected to any other kind of compression device and arranged to generate vibrations in the compression device or be integrated in the compression device. For example the vibrator generator may be a vibrating surface or plate or be connected to a surface or plate, for example a plate constituting the compression device where the plate not necessarily is a piston plate. In one embodiment the compression device and the vibrator generator device is integrated as one device, and the compression is performed by the vibrations. The vibration generator is a piezoelectric or magnetostrictive transducer. In other embodiments, the vibration generator may send the vibrations directly into the paste.

In order to achieve the desired vibration characteristics, the compaction device may comprise a controller unit connected to the vibration generator to control the vibration generator.

The characteristics of the vibrations, such as frequency and amplitude of the vibrations generated by the vibration generator device may be tuned to provide the desired frequency and amplitude in the paste by taking into account the dampening and other effects caused by the connected elements such as piston plate, piston, etc.

In one embodiment the vibration generator device generates high-power vibrations of >1 kW and >8 kHz frequency. In another embodiment, the vibrations may be in the area 20 kHz and above. The compaction device may also be arranged to be able to vary the frequency, or adapt the frequency to the material to be compacted. The compaction device may in this respect comprise sensor devices which can monitor the type of material/paste to be compacted, and the controller unit may be programmed to adapt the vibration characteristics to the present material.

The power source for the compaction device may be any suitable power source, such as a wired power source, or the device may receive power from a connected or adjacent device, or be self-powered.

The invention will now be described in more detail by means of an example and with reference to the accompanying figures.

FIG. 1 illustrates an example of a compaction device used for efficient compaction

FIG. 2 illustrates an example of a compaction device used for hydro fracture generation.

FIG. 1 illustrates a compaction device 10 according to the invention which comprises a compression device 11, in the form of a piston 13 and a connected moving piston plate 14. A vibration generator device 12 is arranged to generate vibrations in the ultrasonic frequency range, and is a sonic or ultrasonically vibrating surface 12 that is affixed to the moving piston-plate 14. The vibration surface 12 can for example comprise a piezoelectric or magnetostrictive transducer.

The vibrations cause fluidization and particle reorganization that allows the mixture to expel fluids (liquids and/or gases) more efficiently, thus providing the desired compression.

The vibration generator device is connected to a power source (not shown) for providing power for generation of the vibrations. In the case that the compaction device is used in a drill tool of the type described in NO312110 (Badger Explorer), the power source can be arranged at the earth's surface and power transferred via the spooled cable, through which power is supplied to the tool and data transferred to surface.

FIG. 2 illustrates an example of a compaction device used for hydro fracture generation. This is a technique to create small fractures (typically less than 1 mm), along which fluids such as gas, petroleum, uranium-bearing solution, and brine water may migrate to the well. Hydraulic pressure is removed from the well, then small grains of proppant (sand or aluminium oxide) hold these fractures open once the rock achieves equilibrium. The technique is very common in wells for shale gas, tight gas, tight oil, and coal seam gas and hard rock wells. This well stimulation is usually conducted once in the life of the well and greatly enhances fluid removal and well productivity, but there has been an increasing trend towards multiple hydraulic fracturing sessions as production declines.

In the example of FIG. 2 the compaction device 20 comprises a compression device 21, for example similar to in FIG. 1. A vibration generator device 22 is arranged to generate vibrations in the ultrasonic frequency range, and is a sonic or ultrasonically vibrating surface 22 that is affixed to the compression device 21. The vibration surface 22 can for example comprise a piezoelectric or magnetostrictive transducer. Vibrations at an injection point during a stimulation are produced by means of the vibrator generation device 22. This reduces the skin friction of the well and provides fractures 23 in the ground into which proppant may migrate. As mentioned above, this will allow wider apertures and hence more productive stimulation. 

1. Subsurface material compaction device comprising: a compression device, a vibration generator device arranged to generate vibrations in the subsurface material, which vibrations are in the ultrasonic frequency range, and a power source connected to the vibration generator.
 2. Compaction device according to claim 1, where the compression device is a moving piston plate.
 3. Compaction device according to claim 2, where the vibration generator device is a sonic or ultrasonically vibrating surface that is affixed to the moving piston-plate.
 4. Compaction device according to claim 1, where the compression device is a plate connected to or integrated in the vibration generator device.
 5. Compaction device according to one of the previous claims, where the vibration generator device is connected to the compression device and arranged to generate vibrations in the compression device.
 6. Compaction device according to claim 5, where the vibration generator device generates high-power vibrations of >1 kW and >8 kHz frequency.
 7. Compaction device according to claim 5, further comprising a controller unit connected to the vibration generator to control the vibration generator.
 8. Compaction device according to claim 5, where the vibration generator is a piezoelectric or magnetostrictive transducer.
 9. Compaction device according claim 5, where the vibrations are in the area 20 kHz and above. 